1. Field of the Invention
This invention relates to a process for producing a sintered body from the particles of a sinterable metal. More particularly, this invention relates to a method of removing a binder from a "green body", as well as to a method of presintering a binder-removed porous body of metal powder particles prior to sintering.
2. Prior Art
A known conventional process for producing sintered bodies comprises the steps of: mixing metal powder particles with a binder composed of a thermoplastic resin, wax, etc.; forming pellets of the mixture; injection-molding the pellets to form a "green body" of a predetermined shape; heating the green body to remove the binder; and sintering the green body in vacuo at elevated temperatures to make an article of a desired shape.
In the step of removing the binder, the green body which has a three-dimensional shape is embedded in an alumina powder or the like in a binder removing furnace and, after the binder is removed, the resulting porous body is taken out of the alumina powder and fed into a separate sintering furnace where it is sintered.
Further, in order to ensure that the alumina powder and jigs that have been fouled during the step of binder removal do not enter the sintering furnace, the porous body must be transferred into a separate vessel before it is charged into the sintering furnace. Holding the porous body, alumina powder and jigs, transferring them, and other associated procedures are hereinafter collectively referred to as "handling".
The binder must be completely removed in the step of binder removal. However, the binder is an organic matter and when the porous body made of metal powder particles from which the greater part of the binder has been removed is heated at elevated temperatures, it is precipitated as carbon which deposits on both the inside and outside surfaces of the porous body and is not removed during the subsequent sintering step. To improve the corrosion resistance, magnetic properties and other characteristics of the sintered body, its carbon content must be reduced to adequately low levels. To meet this need, Japanese Patent Public Disclosure No. 283875/1987 proposed that the atmosphere in the binder removing furnace be saturated with water vapor and the precipitating free carbon be reacted with water vapor to prevent the deposition of carbon on the inside and outside surfaces of the porous body. However, the use of water vapor results in the formation of an oxide film on the surfaces of metal powder particles and an additional step must be provided for reducing this oxide film prior to the sintering step. Further, water vapor cannot be used in a case where the metal powder particles contain Cr and may form a film of chromium oxide and other materials that are very difficult to reduce.